Polishing-amount simulation method for buffing, and buffing apparatus

ABSTRACT

One embodiment of the invention provides a method for simulating polishing amount in a case where a polishing pad of a smaller size than a substrate is used to buff the substrate. The method includes measuring distributions of pressure that is applied from the polishing pad to the substrate according to each overhang amount of the polishing pad relative to the substrate by using a pressure sensor, and correcting the pressure that is used in the polishing amount simulation in accordance with the overhang amounts and the measured pressure distributions.

TECHNICAL FIELD

The present invention relates to a polishing-amount simulation methodfor buffing, and more specifically, to a method for calculating apressure correction value for the polishing-amount simulation forbuffing.

BACKGROUND ART

Semiconductor devices become more and more highly integrated in lateyears, and circuit wiring and integrated devices are accordinglyminiaturized. This trend has generated a need for planarization ofsemiconductor wafer surfaces by polishing the surfaces coated with, forexample, metal films. Planarization methods include polishing by achemical mechanical polishing (CMP) apparatus. The chemical mechanicalpolishing apparatus has polishing members (polishing cloth, a polishingpad, etc.) and holding member (a top ring, a polishing head, a chuck,etc.) for holding a substrate such as a semiconductor wafer. Theapparatus presses the surface (surface to be polished) of the substrateagainst the surface of the polishing member, and brings the polishingmember and the substrate into relative movement while supplying apolishing liquid (abrasive solution, chemical solution, slurry,deionized water or the like) into between the polishing member and thesubstrate. In this manner, the apparatus polishes and planarizes thesurface of the substrate. It is known that the chemical mechanicalpolishing apparatus achieves excellent polishing performance as a resultof the combination of chemical and mechanical polishing actions.

In common chemical mechanical polishing, the to-be-polished surface of asubstrate held by a top ring is pressed against a polishing surfacehaving a larger diameter than the substrate. A polishing table and thetop ring are then rotated while slurry as a polishing solution issupplied onto the polishing surface. The polishing surface and theto-be-polished surface thus come into relative sliding movement, whichpolishes the to-be-polished surface of the substrate.

In late years, the planarizing technology including CMP deals with awide variety of materials to be polished and is required to satisfygrowing demands for high polishing performance (for example, planarity,less polishing damage, and also productivity). Besides, theminiaturization of semiconductor devices creates demands for higherpolishing performance and purity. In such a situation, buffing isoccasionally performed in the CMP apparatus to buff a substrate by meansof a buffing pad of a smaller size than the substrate to be processed.In general, a buffing pad of a smaller size than a substrate to beprocessed is excellent in controllability in that such a pad makes itpossible to planarize the unevenness that is locally generated in thesubstrate, polish only a particular area of the substrate, and adjustthe polishing amount according to the position of the substrate.

To enhance process efficiency and accuracy of planarity in the CMPprocess, it is important to accurately estimate polishing amount andefficiently optimize polishing conditions (such as control parameters ofthe polishing apparatus) based on the estimation. Under the situation,several simulation methods related to CMP have been proposed.

With regard to simulation for the polishing, the estimation of polishingamount is fundamental. In the conventional polishing-related simulationsof various kinds, the polishing amount is estimated by Preston's formulah∝pvt, where h represents polishing rate or polishing amount forpolishing a substrate (to-be-polished object); p represents load orpressure applied to the substrate; v represents contact relativevelocity between a polishing member and the substrate or contactrelative velocity at an area, the polishing amount in which iscalculated; and t represents polishing time. In other words, thepolishing amount is proportional to the product of the pressure p, thecontact relative velocity v, and the polishing time t. In thisspecification, the term “polishing amount” also means polishing amountat each position of the substrate and is referred to also as a polishingprofile.

SUMMARY OF INVENTION Technical Problem

In the buffing is carried out using a buffing pad with a smallerdiameter than a substrate, such as a semiconductor wafer, when theentire surface of the buffing pad is within a periphery of thesubstrate, the pressure applied from the buffing pad to the substrate issubstantially even. As is known, however, when the buffing pad overhangsthe substrate, that is, when the buffing pad partially protrudes overthe substrate, pressure concentration occurs in the vicinity of an edgeof the substrate. For this reason, the simulation of polishing amountbased on Preston's formula requires consideration of effects of thepressure concentration that occurs in the vicinity of the substrate edgewhen the buffing pad overhangs the substrate.

In this light, an object of the present invention is to simulatepolishing amount, taking into account pressure concentration that occursin vicinity of a substrate edge when a small-diameter buffing padoverhangs the substrate to be buffed. Another object of the invention isto determine optimal buffing conditions based on the polishing amountsimulation.

Solution to Problem

A first embodiment provides a method for simulating polishing amount ina case where a polishing pad of a smaller size than a substrate is usedto buff the substrate. The method includes the steps of measuringdistributions of pressure that is applied from the polishing pad to thesubstrate according to each overhang amount of the polishing padrelative to the substrate by using a pressure sensor, and correcting thepressure that is used in the polishing amount simulation in accordancewith the overhang amount and the measured pressure distributions.

A second embodiment provides the method according to the firstembodiment, the method including quantifying the measured distributionsof the pressure applied to the substrate with respect to each overhangamount of the polishing pad relative to the substrate;one-dimensionalizing the quantified pressure distributions with respectto the each overhang amount along a radial direction of the substrate;summing the one-dimensionalized pressure distributions of the eachoverhang amount in the radial direction of the substrate; anddetermining a pressure correction value by dividing the total of thepressure distributions of the polishing pad in the each radial positionof the substrate by distance of the polishing pad on the substrate.

A third embodiment provides a method for simulating polishing amount ina case where a polishing pad of a smaller size than a substrate is usedto buff the substrate. The method simulates polishing amount in a casewhere a part of the polishing pad oscillates over the substrate duringbuffing.

A fourth embodiment provides the method according to the thirdembodiment, wherein the polishing amount is calculated using a pressurecorrection value for correcting an effect of pressure concentration thatoccurs when the polishing pad oscillates over the substrate.

A fifth embodiment provides the method according to the third or fourthembodiment, wherein a buffing condition that is required to achieve agiven target polishing amount is calculated.

A sixth embodiment provides the method according to the fifthembodiment, wherein the buffing condition to be calculated isoscillation velocity of the polishing pad.

A seventh embodiment provides a computer program including a command forcarrying out the simulation according to any one of the third to sixthembodiments.

An eighth embodiment provides a storage medium that stores the computerprogram of the seventh embodiment.

A ninth embodiment provides a buffing apparatus for buffing a substrateby using a polishing pad of a smaller size than the substrate, whereinthe buffing apparatus is configured so that a part of the polishing padoscillates over the substrate during buffing, and the buffing apparatusincludes a simulation section configured to simulate polishing amount ofthe substrate on a given buffing condition.

A tenth embodiment provides the buffing apparatus according to the ninthembodiment, wherein the simulation section performs pressure correctionfor correcting an effect of pressure concentration that occurs when thepolishing pad oscillates over the substrate.

An eleventh embodiment provides the buffing apparatus according to theninth or tenth embodiment, wherein the simulation section calculates abuffing condition that is required to achieve a given target polishingamount.

A twelfth embodiment provides the buffing apparatus according to theeleventh embodiment, wherein the buffing condition to be calculated isoscillation velocity of the polishing pad.

A thirteenth embodiment provides the buffing apparatus according to theeleventh or twelfth embodiment, the buffing apparatus including a sensorfor measuring the polishing amount of the substrate, wherein thesimulation section compares the measured polishing amount of thesubstrate that is buffed on the calculated buffing condition with thetarget polishing amount and, if the target polishing amount is notachieved, calculates a required buffing condition based on the measuredpolishing amount and the target polishing amount.

A fourteenth embodiment provides a method for determining a correctionvalue of pressure that is applied from a polishing pad to a substrate,the correction value being used to simulate polishing amount in a casewhere the polishing pad of a smaller size than the substrate is used tobuff the substrate, wherein the method includes the steps of: measuringdistributions of pressure that is applied from the polishing pad to thesubstrate according to each overhang amount of the polishing padrelative to the substrate by using a pressure sensor; and determiningthe pressure correction value based on the overhang amount and themeasured pressure distribution.

A fifteenth embodiment provides the method according to the fourteenthembodiment, the method including the steps of: quantifying the measureddistributions of the pressure applied to the substrate with respect toeach overhang amount of the polishing pad relative to the substrate;one-dimensionalizing the quantified pressure distributions with respectto the each overhang amount along a radial direction of the substrate;summing the one-dimensionalized pressure distributions of the eachoverhang amount in the radial direction of the substrate; anddetermining a pressure correction value by dividing the total of thepressure distributions of the polishing pad in the each radial positionof the substrate by distance of the polishing pad on the substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a wafer W being buffed with a buffing pad andalso illustrates a graph showing a wiping distance on a wafer position.

FIG. 2 shows pressure concentration that occurs in a wafer edge when thewafer W is buffed with the buffing pad.

FIG. 3 shows layout during measurement of amount of pressure appliedfrom the buffing pad to the wafer W.

FIG. 4 schematically show results of the pressure measurementsillustrated in FIG. 3.

FIG. 5 shows a result of quantification of the amount of pressureapplied from the buffing pad to the wafer W.

FIG. 6 shows a result of quantification of the amount of pressureapplied from the buffing pad to the wafer W.

FIG. 7 is a graph showing a pressure ratio on the wafer position withrespect to each overhang amount.

FIG. 8 is a graph showing a pressure ratio on the wafer position withrespect to each working pressure.

FIG. 9 shows a map of a pressure ratio, in which a horizontal axisrepresents a center position of the buffing pad on the wafer, and avertical axis represents the wafer position.

FIG. 10 is a graph showing a pressure ratio on the center position ofthe buffing pad on the wafer with respect to each wafer position.

FIG. 11 is a graph showing a pressure ratio to the wafer position.

FIG. 12 is a graph showing an example of a polishing profile in whichpressure correction applied during overhang is made.

FIG. 13 is a graph showing polishing amount in a case where buffing isactually carried out on the same conditions except that differentpressures A, B and C are applied.

FIG. 14 is a graph showing pressure coefficients obtained under thepressures A, B and C illustrated in FIG. 13.

FIG. 15 shows examples of target polishing profiles of the wafer.

FIG. 16 shows a state in which an oscillation zone where the buffing padoscillates from the center of the wafer toward the edge of the wafer isevenly divided into eight, and also shows a pressure correction zone andan oscillation velocity correction zone.

FIG. 17 is a schematic view showing a method for calculating theoscillation-velocity correction value according to one embodiment.

FIG. 18 is a schematic view showing a method for calculating theoscillation-velocity correction value according to one embodiment.

FIG. 19 is a schematic view showing a buffing apparatus according to oneembodiment.

FIG. 20 is a flowchart showing the steps of buffing simulation accordingto one embodiment.

FIG. 21 is a flowchart showing the steps of buffing using the buffingsimulation according to one embodiment.

DESCRIPTION OF EMBODIMENTS

Embodiments of a method for simulating polishing amount according to thepresent invention will be explained below with reference to the attacheddrawings. In the attached drawings, identical or similar components areprovided with identical or similar reference marks, and overlappingdescriptions will be omitted in the detailed description. Featuresdescribed in each embodiment can be applied to another embodiment aslong as there is no contradiction therebetween.

When buffing is carried out by oscillating a buffing pad 502 relative toa wafer W (substrate) at a constant rate while the wafer W and thebuffing pad 502 are being rotated at respective constant rotationalspeeds, a wiping distance between the buffing pad 502 and the wafer W isjust as shown in FIG. 1.

FIG. 1 is a schematic side view of the buffing pad 502 buffing the waferW while oscillating on the wafer W. Illustrated under the side view is agraph showing the wiping distance between the buffing pad 502 and thewafer W relative to a position of the wafer W. As illustrated in FIG. 1,when the buffing pad 502 overhangs the wafer W, the wiping distance isdecreased toward an edge of the wafer W.

The wiping distance is a product of a contact relative velocity betweenthe buffing pad 502 and the wafer W, and a processing time. Polishingamount therefore can be calculated from Preston's formula by multiplyingthe wiping distance by pressure that is applied from the buffing pad 502to the wafer W.

When the buffing pad 502 is completely within a periphery of the waferW, the pressure of the buffing pad 502 is considered to be substantiallyeven. When the buffing pad 502 overhangs the wafer W, however, pressureconcentration occurs in the vicinity of the edge of the wafer W asillustrated in FIG. 2.

FIG. 2 is a schematic side view of the buffing pad 502 buffing the waferW while oscillating on the wafer W. Arrows in FIG. 2 denote pressures.The longer arrows represent higher pressures. In FIG. 2, if the buffingpad 502 is located at a position indicated by solid lines, the buffingpad 502 is completely within the periphery of the wafer W, so that thepressure is substantially even as shown by solid arrows. When thebuffing pad 502 oscillates to a position indicated by broken lines,however, the buffing pad 502 overhangs the wafer W, which causes thepressure concentration as shown by broken-line arrows in FIG. 2.

To achieve accurate simulation of the polishing amount using Preston'sformula, therefore, the pressure concentration needs to be taken intoaccount.

One embodiment of the present invention measures pressure distributionswhen the buffing pad 502 overhangs the wafer W and calculates a pressurecorrection value as below.

First, the wafer W is set on a buffing table 400. A sheet-type pressuresensor 1000 (tactile sensor) is placed between the wafer W and thebuffing pad 502. The buffing pad 502 presses against the wafer W withpredetermined force F. The pressure applied to the wafer W is thenmeasured. FIG. 3 is a side view showing layout of the buffing table 400,the buffing pad 502, the sheet-type pressure sensor 1000, and thebuffing pad when the pressure applied to the wafer W is measured. Thepressure distributions are measured with respect to each overhang amountwhile the overhang amount is altered by changing the position of thebuffing pad 502 relative to the wafer W.

FIG. 4 schematically show the pressure distributions measured by thesheet-type pressure sensor. FIG. 4 schematically respectively show, asan example, the pressure distributions measured when the overhangamounts are zero percent, 20 percent, and 40 percent. The percentage ofthe overhang amount here means the ratio of overhang amount of thebuffing pad 502 against the diameter of the wafer W. For example, a 20percent overhang amount means that 20 percent of the diameter of thebuffing pad 502 is in the outer side of the wafer W. If the buffing padhas a diameter of 100 mm, the diameter of the buffing pad protrudes overthe wafer W by 20 mm. In each of FIG. 4, a solid-line circle representsa periphery of the buffing pad. Broken-line arcs each represent a partof the edge of the wafer W.

When the overhang amount is zero percent as illustrated (FIG. 4A), thepressure applied from the buffing pad 402 to the wafer W issubstantially even. When the overhang amount is 20 percent (FIG. 4B),the pressure is increased in the vicinity of the edge of the wafer W anddecreased toward an inner side of the wafer W. Likewise, when theoverhang amount is 40 percent (FIG. 4C), the pressure is increased inthe vicinity of the edge of the wafer W and decreased toward the innerside of the wafer W. In FIG. 4, “HIGH”, “MEDIUM” AND “LOW” indicaterelative pressure magnitudes. The pressure changes in a greater way whenthe overhang amount is 40 percent as compared to when the overhangamount is 20 percent.

After the measurement of two-dimensional distribution of the pressureapplied from the buffing pad 502 to the wafer W, the measured area isdivided into plural divisions, and the measured pressure is quantifiedwith respect to each division.

FIGS. 5 and 6 show, as examples, results of quantification of pressuredistributions measured when the overhang amount in FIG. 4 are zeropercent and 40 percent, respectively. The magnitude of numerical valuesis shown in grayscale. In the drawings, darker grays indicate greaternumerical values. A broken-line arc in each of the drawings represents aperiphery of the wafer W.

As illustrated in FIG. 5, when the overhang amount is zero percent,pressure distributions are substantially even. The divisions lying underthe buffing pad 502 have a constant value, for example, 1.0, and areshown in a uniform gray tone as illustrated.

FIG. 6 shows a result of quantification of the pressure distributionsmeasured when the overhang amount is 40 percent. It is evident from thedrawing that, due to the pressure concentration that occurs in thecircumference of the wafer W, the pressure applied to the circumferenceof the wafer W is high, and the pressure is decreased toward the innerside of the wafer W.

Secondly, the two-dimensional distribution of pressure, which has beenquantified as illustrated in FIGS. 5 and 6, is one-dimensionalized alonga radial direction of the wafer W. More specifically, an average ofnumerical values in a direction of rows (horizontal direction) of FIGS.5 and 6, or on a so-called wafer circumference, is calculated. Thepressure distributions are then one-dimensionalized in the radialdirection of the wafer W (arrow directions in FIGS. 5 and 6). In thisway, a pressure ratio to the radial direction of the wafer W iscalculated.

FIG. 7 is a graph showing a pressure ratio to the radial direction ofthe wafer W in the case where the overhang (OH) amount ranges from zeropercent to 40 percent, inclusive. A horizontal axis represents a radialposition of the wafer W. The wafer W has a diameter of 300 mm. That is,150 mm on the wafer W-position axis indicates the edge of the wafer W.As illustrated in FIG. 7, the pressure ratio increases toward the edgeof the wafer W in proportion to the overhang amount.

The above-described process is repeated, changing the pressure that isapplied from the buffing pad 502 to the wafer W within an actual workingpressure range. As the result, the pressure ratio to the wafer Wposition at each working pressure is obtained.

FIG. 8 is a graph showing the pressure ratio to the wafer W position atthree working pressures as an example. The graph shows a case in whichthe overhang (OH) amount is 20 percent. As is apparent from FIG. 8, evenif the working pressure is changed, there is no significant change inthe pressure ratio to the wafer W position with respect to each overhangamount. Therefore, the same pressure ratio can be applied, regardless ofthe working pressure.

In the next, an approximate expression is made from the pressure ratioto the wafer W position with respect to each working pressure. Anyexpressions, such as a polynomial function, an exponential function,etc., can be used for making the approximate expression.

A pressure ratio map relating to the wafer W position and the buffingpad position on the wafer W is then created from the approximateexpression. FIG. 9 shows a pressure ratio map relating to the wafer Wposition and the center position of the buffing pad on the wafer W. Ahorizontal axis represents the buffing pad position on the wafer W. Thebuffing pad gets closer to the edge of the wafer W toward the right sideof the horizontal axis. A vertical axis represents the position of thewafer W. An upper end of the vertical axis denotes the center of thewafer W, whereas a lower end represents the edge of the wafer W. In FIG.9, the pressure ratio is shown in grayscale. The darker the gray is, thehigher the pressure ratio. The pressure ratio is zero in a region of thewafer W, where the buffing pad 502 does not exist. When the entiresurface of the buffing pad 502 lies completely within the periphery ofthe wafer W (when the overhang amount is zero percent), the pressureratio is 1.0.

Pressure ratios at the center position of the buffing pad on the wafer Ware summed up with respect to each corresponding wafer W position. Inother words, the pressure ratios shown in FIG. 9 are summed up along thehorizontal axis. The total of the pressure ratios with respect to eachwafer W position is divided by the buffing pad diameter (except for theposition where the pressure ratio is zero). A result of the division isa pressure correction value at each wafer W position.

FIG. 10 is a graph showing as an example the pressure ratio to thecenter position of the buffing pad on the wafer W when wafer positionsare 100 mm, 120 mm, 141 mm, and 148 mm. The pressure correction value ateach wafer position can be calculated by calculating area on the graph,which is shown in FIG. 10 with respect to each wafer position, anddividing the area by the buffing pad diameter (except for the positionwhere the pressure ratio is zero). As an example, FIG. 10 includes ashaded area that is the area in the case where the wafer position is 100mm, and shows the buffing pad diameter by an arrow.

FIG. 11 is a graph showing a pressure correction value at each waferposition, which is calculated as explained in the description of FIG.10. As illustrated in FIG. 11, the pressure correction value becomeslarger toward the edge of the wafer.

Once the pressure correction value at each wafer W position isdetermined as described, the pressure correction value can be applied tothe pressure p in Preston's formula h∝pvt. The wiping distance shown inFIG. 1 is a product of oscillation velocity and polishing time.Polishing amount can be calculated by multiplying the wiping distance bythe pressure p. The substantially constant pressure p obtained when thebuffing pad 502 does not overhang the wafer W is multiplied by thepressure correction value at each wafer position, to thereby obtain thepolishing amount taking into account the overhang of the buffing pad502. To be more precise, the wiping distance shown in FIG. 1 ismultiplied by the substantially constant pressure p and the pressurecorrection value shown in FIG. 11, which makes it possible to simulatethe wafer polishing amount, namely, a wafer polishing profile. FIG. 12is a graph showing an example of the polishing profile that is obtainedusing constant oscillation velocity and constant pressure, taking intoaccount the pressure correction applied during overhang.

According to the present invention, since it is possible to simulate thewafer polishing amount taking into account the overhang of the buffingpad, a variety of design parameters of the buffing apparatus can beestimated and optimized by performing the simulation. For example, thesimulation can be performed for optimization of the buffing paddiameter, optimization of rotational speed and rotational speed ratio ofthe wafer and the buffing pad, optimization of the area where thebuffing pad oscillates on the wafer, optimization of the buffing padoscillation velocity distribution, etc. Technology relating to pressuremeasurement, which is disclosed here, is not limited to theabove-described embodiments and can be also applied to a case in which apad of a smaller size than a substrate is pressed against the substrate.

The following description explains the polishing amount simulation usingthe pressure correction value applied during the buffing pad overhang,and also describes creation of buffing conditions.

First, the polishing amount simulation using the pressure correctionvalue applied during the buffing pad overhang will be explained. Asalready discussed, the polishing amount can be basically calculated inaccordance with Preston's formula h∝pvt. In Preston's formula, h is thepolishing rate or polishing amount of a substrate (object to bepolished); p is load or pressure applied to the substrate); v is contactrelative velocity or contact relative velocity of an area, the polishingamount of which is calculated between a polishing member and thesubstrate; and t is polishing time. vt represents a wiping distancebetween the substrate (wafer) and the polishing pad (buffing pad). Thepolishing amount is basically proportional to the wiping distance andthe pressure. However, the actual polishing amount varies withconditions. For this reason, empirical values obtained by actuallyperforming the buffing on various conditions are used as parametercoefficients to improve accuracy in the polishing amount simulation. Thepolishing amount is calculated from a formula, wipingdistance×pressure×pressure correction value×parameter coefficient.

In the present embodiment, the buffing pad is rotated and simultaneouslypressed against the wafer in rotation to polish the wafer. In thisprocess, the buffing pad is oscillated on the wafer to polish the entiresurface of the wafer. The wiping distance can be calculated by asimulator based on software that is separately commercially available.The graph of FIG. 1 shows the wiping distance in the case where thebuffing pad rotating at constant speed is oscillated on the waferrotating at constant speed.

The parameter coefficient is calculated from buffing conditions,features of a dresser, slurry, and a buffing pad that are used for thebuffing, and the like. For example, the parameter coefficient can bedetermined by a polishing amount/pressure ratio as a pressurecoefficient that can be one of the parameter coefficients. FIG. 13 is agraph showing the polishing amount in a case where the buffing isactually carried out on the same conditions except that differentpressures A, B and C are applied. The polishing amount shown in FIG. 13is divided by the pressure to calculate the pressure coefficient. FIG.14 shows pressure coefficients at the pressures A, B and C. In the samemanner, various parameter coefficients can be determined from a slurryflow rate and dilution rate, the features of the dresser and the buffingpad, which are used for the buffing, etc. Actual parameter coefficientscan be acquired as below. The coefficient is assumed as “1” underpredetermined baseline conditions (for example, the pressure is 1 psi;the slurry flow rate is 0.3 L/min; and the buffing pad is provided withhorizontal and vertical grooves in a contact face with the wafer).Change amounts of the polishing amount under conditions other than thepredetermined baseline conditions can be used as various parametercoefficients. The parameter coefficients are previously obtained from atest and stored in a database.

If the pressure correction value applied during the buffing pad overhangis used, the polishing amount can be calculated from the formula, wipingdistance×pressure×pressure correction value×parameter coefficient. FIG.12 shows an example of the polishing amount (also referred to aspolishing profile) that is calculated by the foregoing method.

The following description explains a method for determining buffingconditions for acquiring a desired polishing profile by using thepolishing amount simulation. Consideration is given to a case in whichthe oscillation velocity of the buffing pad is determined as a buffingcondition for acquiring the desired polishing profile on the conditionthat the rotational speed of the buffing pad, the rotational speed ofthe wafer, and the pressure applied from the buffing pad to the waferare given values that are set by user.

FIG. 15 shows examples of target polishing profiles of the wafer. FIG.15 illustrates a polishing profile in which the entire surface of thewafer is planarized, a polishing profile in which the polishing amountis decreased toward the wafer edge, and a polishing profile in which thepolishing amount is increased toward the wafer edge. The buffing padoscillation velocity that is determined in the following description isfor achieving the polishing profile in which the entire surface of thewafer is planarized as a target wafer profile.

First, the polishing profile obtained at constant oscillation velocityis calculated by the foregoing method on the buffing conditions (therotational speed of the buffing pad, the rotational speed of the wafer,the pressure applied from the buffing pad to the wafer, etc.) that areset by user. If the polishing profile is calculated in this way, aplanarized polishing profile cannot be acquired in the vicinity of thewafer edge as seen in FIG. 12 due to the pressure concentration thatoccurs during the overhang and the wiping distance distribution. Tosolve this, time duration in which the buffing pad stays on the wafer isadjusted to obtain such oscillation velocity distributions that theentire surface of the wafer is planarized.

To obtain the oscillation velocity distributions of the buffing pad, thewafer position is divided in a direction from the center of the wafertoward the edge of the wafer. In the present embodiment, the oscillationvelocity is determined with respect to each division so that the entiresurface of the wafer is planarized. FIG. 16 shows as an example a statein which an oscillation zone where the buffing pad oscillates from thecenter of the wafer toward the edge of the wafer is evenly divided intoeight. As other embodiments, the number of divisions may be more or lessthan eight. The oscillation zone does not have to be evenly divided and,for example, may be divided so that the divisions are smaller in thevicinity of the wafer edge.

Since the buffing pad has constant area, a zone in which the oscillationvelocity is corrected differs from the pressure correction zone in whichthe pressure is corrected taking into account the overhang. In concreteterms, as illustrated in FIG. 16, the oscillation velocity correctionzone begins from where the buffing pad oscillates from the center towardthe edge and enters the pressure correction zone.

A method for calculating a correction value of the oscillation velocitycorrection zone will be explained below with reference to FIGS. 17 and18. FIG. 17 shows, in a lower half of the drawing, pressure-correctedpolishing profiles at positions I, II and III of the buffing padoscillating from the center of the wafer toward the edge of the wafer.Curved lines between I and II and those between II and III in FIG. 17correspond to the pressure-corrected polishing profiles at positionsbetween I and II and those II and III of the buffing pad. As discussedbelow, the correction value of the oscillation zone can be calculated bysynthesizing these polishing profiles.

To calculate the oscillation-velocity correction value, oscillationstarting points of the pressure-corrected polishing profiles at therespective positions of the buffing pad are aligned with one another asshown in FIG. 18. An average of the pressure-corrected polishingprofiles at the respective positions of the buffing pad with theoscillation starting points aligned is the correction value of theoscillation velocity. If the buffing pad oscillates on the wafer so thatthe velocity distributions are achieved in accordance with theoscillation-velocity correction value, a planarized polishing profilecan be obtained. The oscillation velocity of the buffing pad may besuccessively controlled so that the oscillation velocity correspondswith the oscillation-velocity correction value. In the presentembodiment, an oscillation range is divided into eight as describedabove, and the oscillation velocity is controlled to be constant withineach division. To that purpose, velocity in each division is calculatedfrom the obtained oscillation-velocity correction value. In theembodiment illustrated in FIG. 18, the oscillation velocity in eachdivision is an average value of the oscillation-velocity correctionvalues within the corresponding division.

It is thus possible to calculate the oscillation velocity of the buffingpad for achieving the target polishing profile (in the foregoingexample, polishing profile for planarizing entire surface) from theuser-set buffing conditions. FIG. 18 shows that the planarized polishingprofile can be obtained if the polishing amount is simulated based onthe user-set buffing conditions and the user-created buffing padoscillation velocity.

A buffing apparatus with the above-described simulation function will bedescribed below. FIG. 19 shows a schematic configuration of a buffingapparatus 300A according to one embodiment. As illustrated in FIG. 19,the buffing apparatus 300A comprises the table 400 on which the wafer Wis set, a head 500 fitted with the buffing pad 502 for processing ato-be-processed surface of the wafer W, and an arm 600 adapted to thehead 500. The buffing apparatus 300A may further comprise a processliquid supply system for supplying a process liquid and a conditioningsection for conditioning (dressing) the buffing pad 502. For clearillustration, the process liquid supply system and the conditioningsection are omitted in FIG. 19. The buffing pad 502 illustrated in FIG.19 has a smaller diameter than the wafer W. As an example, if the waferW has a diameter of 300 mm, the diameter of the buffing pad 502 ispreferably 100 mm or smaller, and more preferably, falls in a rangebetween 60 mm and 100 mm. The process liquid may be at least one of DIW(deionized water), a cleansing liquid, and a polishing liquid such asslurry. The buffing pad 502 is made of, for example, a foamedpolyurethane-type hard pad, a suede-type soft pad or sponge. Whencontrolling or reworking is carried out to reduce dispersion within thewafer surface, a smaller contact area between the buffing pad 502 andthe wafer W makes it possible to deal with a wider variety ofdispersions. In this view, it is desirable that the buffing pad diameteris small. To be more precise, the buffing pad diameter is 70 mm orsmaller and preferably 50 mm or smaller. The kind of the buffing pad 502may be selected as necessary in consideration of the material of thesubstrate and the condition of contamination to be removed. For example,when the contamination is varied under the surface of the substrate, thehard pad that makes it easy to apply a physical force to thecontamination, that is, a pad with high hardness or rigidity, may beused as a pad. When the substrate is a material having a smallmechanical strength, such as a low-k film, the soft pad may be used toreduce damage to the to-be-processed surface. When the process liquid isa polishing liquid such as slurry, a substrate removal rate,contamination removal efficiency, and whether or not damage occurs arenot determined solely by the hardness or rigidity of the pad. The padthereby may be selected as appropriate. The above-listed pads may havesurfaces provided with grooves, such as concentric grooves, X-Y grooves,convoluted grooves, and radiate grooves. It is also possible to providethe pad with at least one hole formed through the pad to supply theprocess liquid through the hole. The pad may be made of sponge-typematerial, such as PVA sponge, into which the process liquid canpenetrate. It is then possible to achieve uniform distributions of aprocess liquid flow within the pad surface and quick discharge of thecontamination removed by the processing.

The table 400 has a mechanism for vacuum-chucking the wafer W and thusholds the wafer W. The table 400 can be rotated around a rotation axis Aby means of a drive mechanism 410. The table 400 may also be configuredto bring the wafer W into angle rotation or scroll motion by means ofthe drive mechanism 410. The buffing pad 502 is fitted to a surface ofthe head 500, which faces the wafer W. The head 500 is rotatable arounda rotation axis B by means of a drive mechanism, not shown. The head 500is capable of pressing the pad 502 against the to-be-processed surfaceof the wafer W by means of a drive mechanism, not shown. The arm 600 iscapable of oscillating the head 500 as shown by arrows C within theradius or diameter of the wafer W. The arm 600 is further capable ofoscillating the head 500 to such a position that the buffing pad 502faces the conditioning section, not shown.

As illustrated in FIG. 19, the buffing apparatus 300A includes a Wet-ITM(In-line Thickness Monitor) 912. The Wet-ITM 912 includes a detectionhead that is located above the wafer W without making contact with thewafer W and moves over the entire surface of the wafer. The Wet-ITM 912thus can detect (measure) film thickness distributions (or distributionsof information about film thickness) of the wafer W. The Wet-ITM isuseful as an ITM for taking measurement during the processing. However,the ITM may be designed to take measurement after the buffing.

A controller 920 is capable of controlling various operations of thebuffing apparatus. The controller 920 controls the pressure applied fromthe buffing pad 502 to the wafer, rotational number of the buffing head500, rotational number of the buffing table 400, oscillation velocity ofthe buffing head 500, etc. The controller 920 receives the filmthickness of the to-be-processed surface of the wafer, which has beendetected by the ITM 912, or a signal corresponding to the filmthickness. The controller 920 includes a user interface and receivesbuffing conditions entered and/or selected by user. The controller 920has a function of calculating the pressure correction of the buffingpad, a function of simulating the polishing amount, and a function ofcalculating optimum oscillation velocity distributions of the buffingpad to achieve the desired polishing profile. The controller 920 maycomprise a dedicated or all-purpose computer. For example, thecontroller 920 can be configured by installing computer programsincluding commands for implementing the above-mentioned various controlfunctions, calculations, and simulations in an all-purpose computer. Thecomputer programs can be stored in an all-purpose storage medium, suchas a hard disc, a CD, and a DVD. A common user interface, such as amonitor, a mouse, a keyboard, and a tablet, may be used as the userinterface of the controller 920.

The buffing apparatus 300A further includes a database (storage section)930 that previously stores the polishing amount corresponding to aplurality of buffing conditions (the pressure of the buffing pad 502against the wafer W, the rotational number of the head 500, and a timeduration in which the buffing pad 502 is in contact with the wafer W).The database 930 also stores preset target film thickness distributionsof the to-be-processed face of the wafer W. The database 930 furtherstores after-mentioned data of various kinds, which are required for thepolishing amount simulation.

FIG. 20 is a flowchart for explaining the steps of the polishing amountsimulation and the optimization of the oscillation velocity in thebuffing apparatus 300A. The polishing amount simulation and theoptimization of the oscillation velocity are carried out by thecontroller 920.

As illustrated in FIG. 20, the buffing simulation is first started (StepS100). In this step, software required for the simulation is activatedin the controller 920.

Polishing conditions for the simulation is then entered (Step S102). Thebuffing conditions include, for example, the size of the wafer as asubstrate, the size of the buffing pad 502, the pressure at which thebuffing pad 502 is pressed against the wafer, the oscillation range ofthe buffing head 500, the rotational number of the buffing table 400,the rotational number of the buffing head 500, and the oscillationvelocity of the buffing head 500. These conditions can be enteredthrough the user interface provided to the controller 920.

In the next step, the pressure correction value is calculated from theentered buffing conditions (Step S104). The pressure correction value isa value that is required when the buffing pad 502 overhangs the wafer.The pressure correction value can be calculated by the above-describedmethod and is as shown in FIG. 11, for example. The pressure correctionvalue is previously determined according to the sizes of the buffing padand the wafer used for the buffing through a test by the steps describedabove, and is stored in the database 930. It is then possible to use thepressure correction value that is required to meet the buffingconditions entered in Step S102.

The polishing amount is then calculated from the buffing conditionsentered in Step S102 and the pressure correction value calculated inStep S104 (Step S106). The polishing amount can be calculated from aformula, wiping distance×pressure×parameter coefficient, using Preston'sformula. As mentioned above, the parameter coefficient is previouslydetermined by a test or the like and stored in the database 930, whichmakes it possible to use the parameter coefficient that is required tomeet the buffing conditions entered in Step S102. The polishing amountresults in, for example, the polishing profile shown in FIG. 12.

The next step calculates difference between the target polishing profileand the polishing profile calculated in Step S106 (Step S108). Thedifference is a polishing-amount correction value. The target polishingprofile may be entered in either Step S102 or Step S108. For example,the polishing profile shown in FIG. 15 may be selected as the targetpolishing profile.

The next step calculates an oscillation correction zone and anoscillation-velocity correction value, which are required to achieve thetarget polishing profile (Step S110). The oscillation-velocitycorrection value can be calculated by the method explained withreference to FIGS. 15 to 18.

In the next step, the buffing conditions entered in Step S102 areupdated based on the oscillation-velocity correction value calculated inStep S110 (Step S112). To be specific, the oscillation velocity isreplaced with the oscillation velocity calculated in Step S110.

The polishing amount is calculated again on the buffing conditionsupdated in Step S112 (Step S114). Since the oscillation velocity hasbeen optimized, the target polishing profile is calculated.

The buffing simulation is then ended (Step S116).

The buffing method using the above-discussed buffing simulation will benow explained. FIG. 21 is a flowchart showing the buffing method usingthe buffing simulation according to one embodiment. The buffing can becarried out using, for example, the buffing apparatus 300A illustratedin FIG. 19.

Once the buffing is started (Step S200), the buffing conditions arefirst set (Step S202). The buffing conditions used here are the buffingconditions created using the polishing amount simulation explained withreference to FIG. 20.

The buffing is started on the buffing conditions set in Step S202 (StepS204).

When the buffing carried out on the set buffing conditions is finished,the film thickness of the wafer that has been buffed is measured by thefilm thickness monitor (ITM 912) (Step S206).

The next step determines whether the polishing profile obtained from thefilm thickness distributions measured by the film thickness monitorconforms to the target polishing profile (Step S208). The determinationcan be made by, for example, comparing the obtained polishing profilewith the target polishing profile in the buffing simulation to check ifthe obtained polishing profile satisfies given conditions.

If Step S208 determines that the target polishing profile is notachieved, buffing oscillation conditions are optimized (S210), and thebuffing is carried out again. The buffing oscillation conditions can beimplemented by the buffing simulation. More specifically, in Step S108associated with the buffing simulation, the polishing-amount correctionvalue is calculated from the difference between the target polishingprofile and the polishing profile measured in Step S206, and theoscillation correction zone and the oscillation-velocity correctionvalue are calculated again. The buffing is carried out again on thebuffing conditions thus obtained.

If Step S208 determines that the target polishing profile is achieved,the buffing is ended (Step S208).

According to another embodiment, closed-loop control in which thedetermination by Step S208 and the optimization by Step S210 take placedoes not necessarily have to be implemented.

According to the present invention, it is possible to simulate the waferpolishing amount taking into account the buffing pad overhang asdiscussed above. Therefore, the estimation and optimization of variousdesign parameters of the buffing apparatus can be made by carrying outthe foregoing simulation.

REFERENCE SIGNS LIST

-   -   400 buffing table    -   410 drive mechanism    -   500 buffing head    -   502 buffing pad    -   600 buffing arm    -   912 ITM (film thickness monitor)    -   920 controller    -   930 database    -   1000 sheet-type pressure sensor    -   W wafer

1-8. (canceled)
 9. A buffing apparatus for buffing a substrate by usinga polishing pad of a smaller size than the substrate, wherein: thebuffing apparatus is configured so that a part of the polishing padoscillates over the substrate during buffing, and the buffing apparatusincludes a simulation section configured to simulate polishing amount ofthe substrate on a given buffing condition.
 10. The buffing apparatus ofclaim 9, wherein the simulation section performs pressure correction forcorrecting an effect of pressure concentration that occurs when thepolishing pad oscillates over the substrate.
 11. The buffing apparatusof claim 9, wherein the simulation section calculates a buffingcondition that is required to achieve a given target polishing amount.12. The buffing apparatus of claim 11, wherein the buffing condition tobe calculated is oscillation velocity of the polishing pad.
 13. Thebuffing apparatus of claim 11, including a sensor for measuring thepolishing amount of the substrate, wherein the simulation sectioncompares the measured polishing amount of the substrate that is buffedon the calculated buffing condition with the target polishing amountand, if the target polishing amount is not achieved, calculates arequired buffing condition based on the measured polishing amount andthe target polishing amount.